Abstract : We reviewed in this chapter, studies of the ERP that have, we think, one characteristic in common. In each case the ERP served as a source of information on the timing or the intensity of an information processing activity whose behavior is not easily monitored by means of observations on overt responses. It would seem that a science of Human Engineering that is interested in developing and testing hypotheses about the internal structure and the operating modes of the human operator would benefit from this additional information. We advocate here the use of the ERP as an analytical tool that can usefully aid in deepening our understanding, and the measurement of, mental workload.